A variety of different systems exist for transporting data signals from and to incoming and outgoing lines, such as in a telephone switching system. One type of such system utilizes a central space division switch under central control which interconnects lines and trunks directly. Another type of system utilizes a combination time and space division switching system, and peripheral control units to which peripherals such as line circuits and trunks are connected. Such systems convert analog signals to data signals and switch them via centralized switches such as circuit switches and message switches, as described in U.S. Pat. No. 4,615,028. Other systems have line circuits and trunks connected to a common bus, and data is sent between them in dedicated channels, such systems being referred to as time division switching systems.
A problem with all such systems is that there is limited bandwidth to provide control signals for the peripheral units. This limited bandwidth has restricted applications and features which otherwise could be provided.
Another family of data communication systems has been described, in which a bus carries data in a ring or U from a bus master. While there are various versions of the structure, typical systems are described in U.S. Pat. No. 4,498,168. In such systems, a bus master initiates a signal to be passed around the bus, and each unit (hereinbelow referred to as a node) which can transmit and/or receive a signal, senses the signal. If the signal is addressed to it, it receives its message, and if it has a signal to transmit to another node or to the bus master, it appends its message to the signal. The signal starts with a "start of train" (SOT) byte and ends with an "end of train" (EOT) byte. There are various forms of such systems, such as the bus being U-shaped, S-shaped, and various techniques are used to ensure that the various nodes have a chance to both receive all signals and transmit to all other nodes and to the bus master, while avoiding signal collisions.
The latter type of system has been found to be unsuitable for use in telephone systems, because in the case of a fault, major disruption to the system occurs. For example, in case of a catastrophic failure of a node, it must be physically removed or bypassed. Such systems are also not easily reconfigured.